1. Field of the Invention
The invention relates to a method for inhibiting the growth of tin whiskers, and more particularly to a method for inhibiting growth of tin whiskers in connecting elements of electronic device packages.
2. Description of the Related Art
Currently, copper or an iron-nickel alloy is the major material used for lead frames of chip packages for providing carriers of chips and electrically connecting chips to print circuit boards. Because the bonding strength between these two kinds of lead frame materials and the print circuit boards is poor, a solder layer is coated on the surface of the lead frame to improve the bonding strength between the lead frame and the print circuit board.
Generally, a tin-lead alloy is used for the solder layer on the surface of the lead frame. Because lead is toxic and harmful to the environment, a lead-free solder is currently used to substitute the conventional tin-lead solder. However, at room temperature, tin whiskers spontaneously grow in the lead-free solder. When the tin whiskers grow to a certain length, two adjacent pins of the lead frame are connected by the tin whiskers and a short occurs between the two adjacent pins of the lead frame. Moreover, while the tin whiskers are grown to a length close to a pitch between the two adjacent pins of the lead frame, a point discharge occurs at the pins of the lead frame and a spark is produced by the point discharge to make the packaged electronic devices fail.
The conventional methods for inhibiting growth of tin whiskers includes three types of methods as shown below:
1. Changing the thickness of the solder layer: the solder layer having a thickness less than 0.5 μm or greater than 20 μm is used for inhibiting the growth of tin whiskers. However, a bridge formation easily occurs in thick solder layers which are used in tiny products and the risk of a short therefore increases. Meanwhile, thin solder layers reduce the functionality of the product and increase the likelihood of corrosion and instability etc.
2. Using an alloy coating layer as a barrier layer: a nickel alloy or a silver alloy layer is formed between a solder layer and a copper substrate to serve as a barrier layer to inhibit the growth of tin whiskers. However, under the effects of thermal expansion, the coefficient difference between the materials of the solder layer, the barrier layer and the copper substrate causes compression stress to form in multi-layered metal structure or an inter-metallic compound layer to induce the growth of tin whiskers
3. Performing an annealing treatment: the structure of a solder layer and a copper substrate are treated with an annealing process at a temperature below the melting point of tin, i.e. 231.89° C. to inhibit growth of tin whiskers. However, an inter-metallic compound (IMC) formation is accelerated in the structure treated by the above mentioned annealing process, at a temperature below the melting point of tin, such that the bonding ability of the solder layer is decreased and the soldering ability of pins in the lead frame is also reduced. Meanwhile, the surface color of the solder layer changes during the annealing process, thus the soldering ability of the solder layer is also reduced.
Therefore, a method for inhibiting growth of tin whiskers which can overcome the above problems and effectively inhibiting the growth of tin whiskers is desired